1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device and a method of manufacturing the device for displaying by reflecting the incident light.
2. Description of the Related Art
Recently applications of liquid crystal display devices in word processor, laptop personal computer, pocket television and others are rapidly advancing. In particular, among the liquid crystal display devices, the reflection type liquid crystal display device for displaying by reflecting the entering light is highly noticed because the power consumption is low since the backlight is not needed, and the design is thin and can be reduced in weight.
Hitherto, for the reflection type liquid crystal display device, the TN (twisted nematic) method and STN (super-twisted nematic) method have been employed, but in these methods, 1/2 of the luminous intensity of natural light is not utilized in display because of the use of the polarizer, and the display is dark.
To solve this problem, display modes for effectively utilizing all of rays of natural light without using polarizer have been proposed. An example of such modes is a phase transition type guest-host method (D. L. White and G. N. Taylor: J. Appl. Phys. 45 4718, 1974). In this mode, the cholesteric-nematic phase transition phenomenon due to electric field is utilized. By combining this method with micro color filter, a reflection type multicolor display is also proposed (Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29/2, 157, 1988).
To obtain a brighter display in the mode not requiring polarizer, it is necessary to increase the intensity of light scattering in a direction vertical to the display screen, for the incident light from all angles. For this purpose it is needed to make a reflector having an optimum reflective characteristic. The above publication discloses a reflector manufactured by roughening the surface of substrate such as glass with abrasive, controlling the surface asperities by varying the time of etching with hydrofluoric acid, and forming a silver foil on the asperities.
In this disclosed reflector, since asperities are formed by injuring the glass substrate with abrasive, asperities of uniform shape are not formed. Another problem is the poor reproducibility of the shape of the asperities, and it is impossible to present a reflection type liquid crystal display device possessing excellent reflective characteristics at high reproducibility by using glass substrate.
FIG. 1 is a plan view of a substrate 2 possessing a thin film transistor (TFT) which is a switching element used in active matrix method, and FIG. 2 is a sectional view of XI--XI in FIG. 1. On an insulating substrate 2 of glass or the like, plural gate bus wirings 3 made of chromium, tantalum or the like are disposed parallel mutually, and gate electrodes 4 are branched off from the gate bus wirings 3. The gate bus wirings 3 function as scanning lines.
Covering the gate electrodes 4, a gate insulating film 5 made of silicon nitride (SiNx), silicon oxide (SiOx) or the like is formed on the entire surface of the substrate 2. On the gate insulating film 5 above the gate electrode 4, a semiconductor layer 6 composed of amorphous silicon (a-Si), polycrystalline silicon, CdSe or the like is formed. At one end of the semiconductor layer 6, a source electrode 7 made of titanium, molybdenum, aluminum or the like is superposed. At the other end of the semiconductor layer 6, same as the source electrode 7, a drain electrode 8 made of titanium, molybdenum, aluminum or the like is superposed. At the opposite end of the drain electrode 8 against the semiconductor layer 6, a picture element electrode 9 made of ITO (indium tin oxide) is superposed.
As shown in FIG. 1, a source bus wirings 10 crossing the gate bus wirings 3 across the gate insulating film 5 is connected to the source electrode 7. The source bus wirings 10 function as signal lines. The source bus wirings 10 are also made of the same metal as the source electrode 7. The gate electrode 4, gate insulating film 5, semiconductor layer 6, source electrode 7, and drain electrode 8 composed a TFT 1, and this TFT 1 possesses the function of switching element.
To apply the substrate 2 possessing the TFT 1 shown in FIG. 1 and FIG. 2 in a reflection type liquid crystal display device, it is necessary to form the picture element electrode 9 by using a metal possessing light reflectivity such as aluminum and silver, and form the gate insulating film 5 or asperities thereon. Generally, it is difficult to form tapered asperities uniformly on an insulating film made of inorganic matter.
FIG. 3 is a plan view of a substrate 12 possessing a TFT 11 used in active matrix method, and FIG. 4 is a sectional view of XII--XII in FIG. 2. On an insulating substrate 12 made of glass or the like, plural gate bus wirings 13 made of chromium, tantalum or the like are disposed parallel mutually, and gate electrodes 14 are branched off from the gate bus wirings 13. The gate bus wirings 13 function as scanning lines.
Covering the gate electrodes 14, a gate insulating film 15 made of silicon nitride, silicon oxide or the like is formed on the entire surface of the substrate 12. On the gate insulating film 15 above the gate electrode 14, a semiconductor layer 16 made of a-Si or the like is formed. At both ends of the semiconductor layer 16, contact layers 17 made of a-Si or the like are formed. On one contact layer 17, a source electrode 18 is superposed, and on the other contact layer 17, a drain electrode 19 is superposed. Source bus wirings 23 functioning as signal lines crossing the gate bus wirings 13 across the gate insulating film 15 are connected to the source electrode 18. The gate electrode 14, gate insulating film 15, semiconductor layer 16, contact layer 17, source electrode 18, and drain electrode 19 compose a TFT 11.
Further, an organic insulating film 20 possessing plural bumps 20a and having a contact hole 21 on the drain electrode 19 is formed. On the organic insulating film 20, a reflection electrode 22 is formed, and the reflection electrode 22 is connected to the drain electrode 19 through the contact hole 21.
When the organic insulating film 20 is formed on the substrate 12 having such TFT 11 formed thereon, the bumps 20a may be easily formed on the surface of the organic insulating film 20 by employing the etching method, and by forming the reflection electrode 22 on the organic insulating film 20 having the bumps 20a, the reflection electrode 22 having asperities may be easily formed.
As shown in FIG. 1 and FIG. 2, when forming the reflection electrode 9 and source bus wirings 10 on the gate insulating layer 5, a gap 9a is formed so as to prevent conduction between the reflection electrode 9 and source bus wirings 10. However, as shown in FIG. 3 and FIG. 4, such gap 9a is not needed when the source bus wirings 23 are formed on the gate insulating film 15, and the reflection electrode 22 on the organic insulating film 20.
To enhance the luminance of display, it is desired that the reflection electrode 22 be as large as possible. In FIG. 3 and FIG. 4, therefore, the end portion of the reflection electrode 22 is formed on the source bus wirings 23 through the organic insulating film 20.
Since the organic insulating film 20 possesses the bumps 20a, if etching failure of contact of the bottom portion 20b of the adjacent bumps 20a on the source bus wiring 23 occurs, insulation by the organic insulating film 20 is not achieved, and insulation failure may occur between the source bus wirings 23 and the reflection electrode 22 formed on the organic insulating film 20.
When patterning, meanwhile, the reflection electrode 22 in order to form the organic insulating film 20 possessing bumps 20a on the entire surface of the substrate, asperities may be formed in the end portion of the reflection electrode 22 due to bumps 20a, which may result in defective patterning of the reflection electrode 22.
Furthermore, when the reflection electrode 22 is formed through the organic insulating film 20 on the semiconductor layer 16 of the connecting part on the gate electrode 14 which is a distribution electrode formed on the substrate, the signal to be applied to the reflection electrode 22 is applied to the semiconductor layer 16, and the reflection electrode 22 spuriously acts like the gate electrode 14, and a channel is formed in the interface between the reflection electrode 22 and semiconductor layer 16, thereby lowering the characteristic of the TFT 11. Besides, a large parasitic capacity is generated between the gate electrode 14 and reflection electrode 22. These phenomena cause to lower the display grade.